1. Field of the Invention
This invention relates to a method of manufacturing an acoustic diaphragm used in sound instruments such as speakers and microphones.
2. Description of the Prior Art
In recent years, there is a progress in the achievement of digital sound instruments that makes much severer the performance required in diaphragms for speakers or the like. For example, it is required for the diaphragms to be less deformed by an external force to cause less sound distortion, and be capable of reproducing sounds over a wide pitch and giving a clear-sound quality. For this purpose, they are required to be light-weight, and yet to have an excellent elasticity or stiffness. These requirements can be summarized as follows, as specific conditions for their physical properties.
(1) They have a large Young's modulus (E). PA1 (2) They have a small density (.rho.). PA1 (3) They have a large sound velocity (propagation velocity V of sound waves). PA1 (4) They undergo an appropriate internal loss (tan.delta.) of the vibration. PA1 (5) They have a large strength. PA1 (6) They can be molded in any desired form.
Between V, E and .rho. in the foregoing, there is the relationship of: EQU V=(E/.rho.).sup.1/2
In addition to these conditions, it is needless to say that they are required to be manufactured with ease and be stable to external conditions such as heat and humidity.
As material for the diaphragms, paper, plastic, aluminum, titanium, beryllium, boron, silica, etc. have been hitherto used. These have been used alone, or in combination with glass fiber, carbon fiber or the like in the form of a composite. They have been also used in the form of a metal alloy. Paper and plastic, however, are not satisfactory in the characteristics such as Young's modulus, density and sound velocity. In particular, they have extremely poor frequency characteristics in a high frequency band, and it has been difficult to obtain a clear-sound quality when they are used as diaphragms for tweeters or squawkers. Aluminum, magnesium, titanium, etc. can each attain a reasonably good sound velocity, but have so small an internal loss of vibration that they may cause a phenomenon of high-frequency resonance. Thus, these materials have been able to achieve only unsatisfactory characteristics for high-frequency diaphragms. As for boron, beryllium, etc., they have better physical properties than the materials discussed above, and hence can attain a good sound quality required for the diaphragms. The boron, beryllium, etc., however, are disadvantageous in that they are very expensive and also have poor workability.
Diaphragms comprised of a carbon material are being developed with the aim of diaphragms that have overcome the disadvantages involved in the conventional diaphragms, have superior high-frequency wave characteristics and also can reproduce a tone with a good quality. This makes the most of the excellent physical properties inherent in carbon (graphite) so that it can be used as a diaphragm. Such a material for diaphragms can be obtained by the methods as exemplified below.
(1) A method in which a graphite powder and a polymeric resin are combined in a composite form.
(2) A method in which a graphite powder and a polymeric resin are combined in a composite form, followed by sintering into a graphite-carbon composite.
(3) A polymeric compound is carbonized by heat treatment.
Of these methods, a typical diaphragm obtained by the method (1) is a diaphragm of a composite type comprising a matrix vinyl chloride resin and a graphite powder. This is known to be a diaphragm having superior properties. The method (2) may more specifically include a method in which a graphite powder is mixed in a liquid crystal component of a crude oil decomposed pitch and then the mixture is heated to effect carbonization, and a method in which a binder that bounds graphite powder is added to a graphite powder, which is then heated to effect carbonization. In the latter case, a method is known in which, in the carbonization of the binder, a monomer or prepolymer of a thermosetting resin is heated together with a thermoplastic resin having functional groups capable of decomposing upon heating and mutually reacting to crosslinkingly cure. These methods have been developed with the aim of preventing shrinkage or deformation from occurring when the heat treatment is carried out, and can give diaphragms with superior characteristics.
However, the diaphragms obtained by the method (1) have so poor humidity and temperature characteristics that their vibration characteristics may become seriously deteriorated at temperatures of 30.degree. C. or above.
The method (2) requires a complicated process for the manufacture, and has been very disadvantageous when diaphragms are mass-produced in an industrial scale. That is, for example, from the viewpoint of the manufacturing process, what is questioned is that very complicated steps such as high-temperature treatment and solvent fractionation or extraction must be provided so that the crude oil decomposed pitch used as a starting material and the liquid crystal component thereof are obtained in an industrial scale. From the viewpoint of the mass production, what is questioned is that a high-level technique is required such that the graphite powder and the binder resin are thoroughly kneaded using a kneader with a high shear force so that the graphite crystals cleaved by mechanochemical reaction and the binder resin are dispersed in a mutually strongly combined state and the crystal planes of the graphite are oriented in the direction of the surface of a sheet. In addition, although the diaphragms obtained by these methods have very superior characteristics that have not been hitherto attained, they have slightly poorer characteristics than the characteristics obtainable by beryllium, considered to be of a highest level at present, and have an elasticity that is no match for the theoretical elasticity 1,020 GPa of graphite monocrystals.
In the method (3), so good characteristics as expected at first were not obtainable since all conventional plastics films are hardly graphitizable materials. Moreover, this method have been disadvantageous in that plastic materials used have so low a carbon yield and undergo so large a dimensional shrinkage that the products are often deformed or cracked. That is, it has been difficult to obtain diaphragms that can be molded into any desired form, can endure thorough quality control and also have excellent characteristics.
In order to eliminate the disadvantages of the method (3), the present inventors have proposed that a film comprising a specific polymeric compound is heated in an inert gas to form a graphite film, and the resulting graphite film is used as a diaphragm (Japanese Laid-open Patent Application No. 3-085899). This is based on a recent discovery that some of condensation polymers can be graphitized by a high-temperature heat treatment, which are called graphitizable materials.
The graphite film obtained by this method, however, has had the problems that it has a small thickness and also may still undergo so large a dimensional shrinkage at the time of heating as to cause deformation or cracks, and hence it has been impossible to mold the graphite film (or sheet) into any desired form. As a result, this method has been disadvantageous in that only a flat diaphragm can be produced and also the diaphragm can not have a strength high enough to be used as an acoustic diaphragm.